The present invention principally relates to improvements in negative pressure boosters in which a brake master cylinder of an automobiles is boosted and operated under negative pressure, and more particularly to a booster comprising a booster piston which divides the interior of a booster shell into a first operating chamber internally communicated with a negative pressure source and a second operating chamber, and a valve cylinder protruded at the rear of the booster piston and having a first annular valve seat disposed at the front portion thereof, the valve cylinder slidably receiving a valve piston having a second annular valve seat encircled by said first valve seat and connected to an input rod and including a cylindrical resilient valve body having a valve portion biased to be seated on said first and second valve seats and inside of which is open to the atmosphere, whereby when said valve piston is moved forward, said second operating chamber and said inside of said valve body are brought into communication through a clearance formed between said second valve seat and said valve portion whereas when said valve piston is retracted, said first and second operating chambers are brought into communication through a clearance formed between said first valve seat and said valve portion, and said booster piston is connected through a resilient piston to an output piston communicated with an output rod so that operative reaction of said output piston causes the resilient piston to give rise to compressed deformation, a part of the compressive force being fed back to the valve piston and input rod through a reaction piston to impart a sense of operation to an operator.
In the booster of the type as described, the resilient piston functions as if it were a non-compressed fluid to transmit the operative reaction of the output piston to the reaction piston. Thus, the above-mentioned booster has advantages that it is possible to determine simply the boost ratio from the ratio of area between the output piston and the reaction piston, and the device is easy in design and simple in construction. In prior art boosters, however, when the input rod is retracted (at this time, the valve piston is considerably retracted), the resilient piston undergoes inflated deformation toward the reaction piston through a sliding stroke portion of the valve piston. Since such inflated deformation is produced in great amount, the resilient piston is apt to become fatigued and failed in a short period of time in addition to poor durability.
It is therefore an object of the present invention to provide a negative pressure booster which can overcome the difficulties noted above with respect to prior arts.